Switch



Nov. 13, 1951 F, EE 2,574,869

SWITCH Filed Dec. 9, 1948 INVENTOR JOHN D E GREEN 8 ATTORNEYS eirect of bending the Patented Nov. 13, 1951 SWITCH John Dudley Francis Green, Cambridge, England, assignor to Pye Limited, Cambridge, England,

a British company Application December 9, 1948, Serial No. 64,305 In Great Britain December 10, 1947 8 Claims. (Cl. 200-122) This invention relates to electric switches.

An object of the invention is to provide a snap-action time-delay switch which is useful for controlling the high tension supply to thermionic tubes as well as for other purposes. Another object is to provide a thermal timedelay switch which is compensated for by changes in ambient temperature. Another object is to achieve the foregoing objects with a switch mechanism which is simple and economical to construct and manufacture and is reliable and durable in service.

In order that the invention may be more clearly understood, reference is made to the accompanying drawing in which Figs. 1 and 2 are diagrams illustrating the principle of operation of the switch.

Fig. 3 is a schematic side view of one embodiment of a thermal time-delay switch according to the invention.

Referring to Fig. 1, if a spring strip ID of bowed cross-section is held fast at one end by fixture to a suitable anchorage i I, and if a bearing or stop represented by arrow i2 is positioned to engage the convex side of the strip ID at a point spaced from the anchorage il, then the application to the concave side of the stri of a force represented by arrow i3 and applied at some point along the portion M of the strip between the point of application of the bearing i2 and the free end of the strip, will have the portion M of the strip longitudinally about the bearing I! in opposition to the spring resistance which the bowed cross-section of the strip offers to such longitudinal bending, until ultimately a condition in the bent portion I 4 of the strip is reached where the resistance of the bowed cross-section is unable further to withstand the bending force, with the result that the bowed cross-section of the strip abruptly collapses to a flattened form in the region of the bearing l2 and by virtue of that collapse the portion H of the strip suddenly flexes about the bearing i2 and thus executes an abrupt, rapid and relatively large movement in the direction of bending, as is graphically represented in Fig. 2. In that figure, the flexure of the portion II is exaggerated for the purpose of illustration. Only a small advancing movement of the point of application of the bending force up to that position is critical for causing the abrupt flexing of the portion l4 of the strip-and conversely, only a small retraction of the point of application of the bending force is needed to permit the collapsed cross-section of the strip to regain its bowed form and in so doing quickly re-ilex the portion H of the strip. The bending force l3 may be applied at any point along the portion id, but the nearer the point of application is to the bearing II, the smaller will be the displacement of the point of application of the force I3 needed to flex the strip.

An example of a thermal switch embodying the invention will now be described with reference to Fig. 3. In radio and other apparatus employing thermionic tubes, it is customary to provide a time-delay switch for automatically connecting the high tension supply to such tubes only after the lapse of a predetermined time interval from switching on the apparatus, which is sufficient to allow the filaments or heaters of the tubes to attain their working temperature. The thermal time-delay switch schematically depicted in Fig. 3 may be employed for this purpose, as well as for other purposes. In this embodiment of the invention, the switch comprises an insulating base plate on which is mounted an insulating block or pillar ii to which one end of a spring strip 62 of bowed cross-section is rigidly fixed with its convex side facing the base plate 60, the free end of the strip carrying a contact 63. Overlying the strip is a bi-metallic strip element 64 in the form of a bridge having one end portion is clamped on to the fixed end of the strip 62 by means of a fixing bolt 66 which passes through the portion ll of the bi-metallic strip, through the strip 62 and the block GI and secures that assembly'to the base plate 60. The bi-metallic strip 64 has an intermediate portion 6! spaced from the concave side of the strip 62 and a free end portion 68 which bears with its tip against the free end of the strip 62. Another strip 69, also of bridge form, has one end portion 10 secured to the block Si by means of the bolt 66 against the convex side of the strip 62, an intermediate portion ll spaced from the strip 62 and a free end portion 12 which bears against the convex side of the strip 62 at a point intermediate the block II and end 68 of strip 64. .A stationary contact I! spaced from contact 63 is carried on the free end of a further strip 14 which is fixed at its other end to the base plate 60. The bi-metallic strip 64 is provided with a heater winding II, c. of nickel chrome, the turns of which are electrically insulated fronTstrip 64, e. g., by a glass sleeving, and one end of the heater I8 is connected to a terminal 16 on the base plate 6., the other end of the heater being connected to the end 68 of the bi-metallic strip H. The heater circuit is completed through the strips 64, 62 and bolt 66 to a terminal 11 on the base plate 60. The fixed end of the strip 14 is connected to another terminal 18 on the base plate 60.

In the use of the switch, the circuit to be controlled by the switch is connected to theterminals l1 and 18, thereby connecting the contacts 63 and '13 in such circuit. The terminals 16 and I! are connected to a circuit for supplying heating current to the winding 15. When the heater circuit is closed and the bi-metallic strip 64 is consequently heated, it bends towards the strip 62 under thermal expansion and thereby causes its end 68 to build up a pressure on the end of the strip 62 until the latter is caused to flex suddenly, in the manner described with reference to Figs. 1 and 2, and so close the contact 63 onto the contact 13. The time delay between the closing of the heater circuit and the closing of the switch may be adjusted by adjustment of the initial pressure of the bearing 12 on, or its spacing from, the strip 62, and the spacin of the stationary contact 13 from the contact 63. this purpose, adjusting screws 79 and 80 are threaded through the base plate 60 and bear against the strips 69 and 14 respectively, these strips being made resilient so that they maintain engagement with their screws.

It will be appreciated that a change in ambient temperature would alter the time-delay setting of the switch, since such change would cause the bi-metallic strip 64 to bend towards or away from the strip 62 (with no current flowing in the heater winding 15) according as the ambient temperature rises or falls respectively, and to an extent depending upon the magnitude of the ambient temperature change, so that when the bi-metallic strip is heated by means of the heater winding, a shorter or longer time would be required for it to close the switch.

If a rise in ambient temperature occurred sufflcient to cause the bi-metallic strip 64 to snap over the strip 62, the switch contacts would close without operation of the heater 15. Accordingly, provision is made to compensate for changes in ambient temperature so as to render the time-delay settin of the switch independent of such changes. For this purpose, both the strips- 69 and 14 are constituted by bi-metallic strips of the same thermal characteristics as the main bi-metallic strip 64 so that any change in ambient temperature affects the three strips equally and causes them to move in the same direction by the same amount. Thus, upon an'ambient temperature rise with the heater l5 inoperative, the strip 69 bends and moves the bearing 12 away from the contact-carrying strip 62 by the same amount as the bi-metallic strip 84 bends and moves its end 68 towards the strip 62, so that such movement of the end 68 cannot abruptly flex the strip 62, and consequently the time required to heat the main bi-metallic strip 64 by means of the heater winding 15 sufflciently to cause it to efiect the abrupt flexing of the strip will remain constant. Movement of strip 64 towards, and of strip 69 away from, the strip 62 as a result of ambient temperature rise would cause the gap between the contacts 63 and I3 to become less, and if the ambient temperature rise were sufficient, the contacts would close without the strip 62 having snapped over and, what is more important, without current flowing in the heater 15. This is prevented by the strip H being made a bi-metallic strip as above mentioned so that it bends and moves the con- For tact 13 away from contact 63 at the same rate as the latter is moved towards contact 13, and thus the gap between the contacts will be kept constant.

While a specific embodiment of the invention has been described in the foregoing, this is merely illustrative and the invention is not limited thereto since various other forms of the invention may be constructed to suit the requirements of any particular use. For example, whereas the switch of Fig. 3 is normally open and is closed upon actuation, it will be evident that'change-over switches which, upon actuation, make one circuit and break another, may be constructed embodying the mechanism according to this invention. Also, the invention is applicable also to relays.

I claim:

1. A thermal switch device comprising a base member, a bowed-section spring strip, means for rigidly supporting one point of said strip relative to said base member, two bearing members in contact with said strip at points spaced from each other and from said one point longitudinally of said strip, said bearing members being so arranged as to be capable in conjunction with said means to produce a moment tending to bend said strip transversely and oppositely to its bowed section, two thermo-expansion means each interposed between one of said bearing members and said base member for causing said bearing members to increase and decrease the bending force on said strip upon a rise in temperature, said thermo-expansion means being so dimensioned in relation to each other that the increase in said moment produced by one thermo-expansion means under any variation of temperature is substantially compensated for by the decrease in said moment produced by the other thermo-expansion means under the same variation of temperature, and means for producing a difference in temperature between the two thermo-expansion means whereby to cause the strip to suddenly change its curvature.

2. An electrical switch device comprising a base member, a bowed-section spring strip, means for rigidly supporting one point of said strip relative to said base member, two bearing members in contact with said strip at points spaced from each other and from said one point longitudinally of said strip, said bearing members being so ar-- ranged as to be capable in conjunction with said means to produce a moment tending to bend said strip transversely and oppositely to its bowed section, two thermo-expansion means each interposed between one of said bearing members and said base member for causing said bearing members to increase and decrease the bending force on said strip upon a rise in temperature, said thermo-expansion means being so dimensioned in relation to each other that the increase in said moment produced by one thermo-expansion means under any variation of temperature is substantially compensated for by the decrease in said moment produced by the other thermo-expansion means under the same variation of temperature, means for producing a difference in temperature between the two thermo-expansion means whereby to cause the strip to suddenly change its curvature, a movable switch contact carried by said strip for operation by the sudden flexing thereof, a second switch contact for co-operation with said movable switch contact, and auxiliary thermo-expansive means for moving the second contact in accordance with temperature variations to between the movable and second switch contactsdue to simultaneous and equal alterations in the temperatures of the two thermo-expansion means.

3. A thermal switch device comprising a base member. a bowed-cross-section spring strip fixed to said base member at one point, a first bearing constructed to act on said strip at a bearing point spaced from said one point to urge the strip outwardly of its curvature and substantially normally to the central tangential plane thereof, a second bearing constructed to act upon the convex side of the strip at a second bearing point intermediate said one point and said first bearing point in a direction substantially opposite to the action of said first bearing, first thermo-expansion means for moving said first bearing in its direction of action in response to variations of temperature, second thermo-expansion means for moving said second bearing in the same direction in response to similar variations in temperature, said two thermo-expansion means being so dimensioned that the relative alignment of said three points is substantially unaffected by any common variation of the temperatures of said two thermo-expansion means, and means for producing a temperature difference between said two thermo-expansion means and thereby causing the strip to flex suddenly adjacent said second bearing.

4. A thermal switch device comprising a base member, a bowed-cross-section spring strip fixed to said base member at one point, a first bearing constructed to act on said strip at a bearing point spaced from said one point to urge the strip outwardly of its curvature and substantially normally to the central tangential plane thereof, a second bearing constructed to act upon the convex side of the strip at a second bearing point intermediate said one point and said first bearing point in a direction substantially opposite to the action of said first bearing, first thermo-expansion means for moving said first bearing in its direction of action in response to variations of temperature, second thermo-expansion means for moving said second bearing in the same direction in response to similar variations in temperature, said two thermo-expansion means being so dimensioned that the relative alignment of said three points is substantially unaffected by any common variation oi the temperatures of said two thermo-expansion means, and electric heating means associated with one of said thermoexpansion means for producing a temperature difierence between said two thermo-expansion means and thereby causing the strip to flex suddenly adjacent said second bearing.

5. A thermal switch device comprising a base member, a bowed-cross-section spring strip fixed to said base member at one point, a first bearing constructed to act on said strip at a bearing point spaced from said one point to urge the strip outwardly of its curvature and substantially normally to the central tangential plane thereof. a second bearing constructed to act upon the convex side of the strip at a second bearing point intermediate said one point and said first bearing point in a direction substantially opposite to the action of said first bearing, a bimetallic element for moving said first bearing in its direction of action in response to variations of temperature, a second bimetallic element for moving said second bearing in the same direction in response to similar variations in temperature, said two bimetallic elements being so dimensioned that the relative I alignment of said three points is substantially unafiected by any common variation of the temperatures or said two bimetallic elements and an electric heating resistance associated with one of said bimetallic elements for producing a temperature diil'erence between said two bimetallic elements and thereby causing the strip to suddenly change its curvature adjacent said second bearing at predetermined values or the temperature difi'erence between said two bimetallic elements.

6. A time switch device comprising a base membet, a bowed-cross-section spring strip fixed to ,said base member at one point, a first bearing constructed to act on said strip at a bearing point spaced from said one point to urge the strip outwardly of its curvature and substantially normally to the central tangential plane thereof, a second bearing constructed to act upon the convex side of the strip at a second bearing point intermediate said one point and said first bearing P int in a direction substantially opposite to the action of said first bearing, a first bimetallic strip for resiliently moving said first bearing in its direction of action in response to variations of temperature, a second bimetallic strip for moving said second bearing in the same direction in response to similar variations in temperature, said two bimetallic strips being so dimensioned that the relative alignment of said three points is substantially unaiiected by any common variation of the temperatures of said two bimetallic strips and electric heating means associated with said first bimetallic strip for individually heating said first bimetallic strip and thereby causing the bowedsection strip to flex suddenly adjacent said second bearing after a predetermined time, when a predetermined voltage is applied to said heating resistance, said predetermined time being substantially independent of variations in ambient temperature.

7. An electrical switch device comprising a base member, a bowed-cross-section spring strip fixed to said base member at one point, a first bearing constructed to act on said strip at a bearing point spaced from said one point to urge the strip outwardly of its curvature and substantially normally to the central tangential plane thereof, a second bearing constructed to act upon the convex side of the strip at a second bearing point intermediate said one point and said first bearing point in a direction substantially opposite to the action of said first bearing, a first bimetallic strip for resiliently moving said first bearing in its direction of action in response to variations of temperature, a second bimetallic strip for moving said second bearing in the same direction in response to similar variations in temperature, said two bimetallic strips being so dimensioned that the relative alignment of saidthree points is substantially unaffected by any common variation of the temperatures of said two bimetallic strips, electric heating means associated with said first bimetallic strip for individually heating said I first bimetallic strip and thereby causing the bowed-section strip to fiex suddenly adjacent said second bearing after a predetermined time, when a predetermined voltage is applied to said heating resistance, said predetermined time being substantially independent of variations in ambient temperature, a movable switch contact carried by said bowed-section strip at a point further spaced from said first bearing than said one point, a bimetallic strip connected to the base of the switch device and a second switch contact for co-opcration with said movable switch contact, said second contact being carried by said last-mentioned bimetallic strip ior movement in substantially the same direction as said bearings under similar variations oi! temperature, whereby said second contact is caused to substantially follow the movement of said movable contact produced by variations of ambient temperature.

8. An electrical switch device comprising a base member, a bowed-cross-section spring strip fixed to said base member at one point, a first bearing constructed to act on said strip at a bearing point spaced from said one point to urge the strip outwardly of its curvature and substantially normally to the central tangential plane thereof, a second bearing constructed to act upon the convex side of the strip at a second bearing point intermediate said one point and said first bearing point in a direction substantially opposite to the action of said first bearing, first thermo-expansion means for moving said first bearing in its direction of action in response to variations of temperature, a second thermo-expansion means for moving said second bearing in the same direction in response to variations in temperature, said two thermo-expansion means being so dimensioned that the relative alignment of said three points is substantially unaffected by any common variation of the temperatures of said 7 ing a temperature diflerence between said two file of this patent:

UNITED STATES PA Number Name Date 1,581,12 Humphrey Apr. 20, 1926 25 1,900,647 Hoopes i Mar. '1, 1933 2,054,558 Dederick Sept. 15, 1936 2,208,432 Samuels July 16, 1940 2,399,123 Jordan Apr. 23, 1%6

thermo-expansion means and thereby causing the strip to suddenly change its curvature adjacent said second bearing, a movable switch contact operatively connected to said strip for operation by the sudden change of curvature of the strip, a second switch contact for co-cperation with said movable contact, and thermo-expansive means interposed between said base and said second contact and so dimensioned as to cause said second contact to substantially follow the movements of said movable contact due to the simultaneous movement of said first and second bearings under variations of ambient temperature.

JOHN DUDLEY FRANCE GREEN.

REFERENQES crrnn The following references are of record in the 

